1. Field of the Invention
The present invention relates to a road surface detecting device for detecting the condition of a road surface on which a vehicle is running.
2. Description of the Prior Art
When a vehicle runs on good roads or streets or the like, it is desired to absorb vibrations of small amplitude from the road surface by decreasing the damping force of a suspension of the vehicle conversely, and when a vehicle runs on bad roads such as mountain roads, it is desired to damp vibrations of large amplitude from a road surface, by increasing the damping force of the suspension of the vehicle.
In order to automatically control the damping force of the suspension as described above, a detecting device for detecting the condition of the road surface on which the vehicle runs is required. Such a detecting device has been proposed as a device for deciding the road condition from the variations of vehicle speed, that is from the acceleration or the deceleration of the vehicle (U.S. Pat. No. 4,506,909).
Since the variation of vehicle speed accurately corresponds to road conditions, the above described device exhibits excellent performance. However, the above described device has a problem. Namely, when a vehicle passes partially uneven portions on a good road, such as joints of a road, the variation of vehicle speed is temporarily increased. This results in the above described device erroneously interpreting these conditions as bad road conditions and erroneously increasing the damping force of a suspension of the vehicle. Consequently, the vehicle provides an uncomfortable ride.
Furthermore, when the air pressure of vehicle tires is changed over a large range and when the running speed of a vehicle is also changed, the acceleration or the deceleration is affected thereby. This results in conditions of roads being erroneously detected.